Nowadays images can be projected onto a screen or viewed directly on large displays such as LED or LCD displays, e.g. for home theatre applications. The contrast in a highly lit environment is one of the advantages of a display like an LCD. Projection systems on the other hand offer larger screens with a lower weight and ease of installation. Hence, to improve the performance of projection system it would be preferred to reduce the reflections of ambient light falling onto the screen.
Where multiple screens or curved screens are used in projection system, cross-screen reflections can occur, e.g. in simulation setups or in an immersive screen setup like the Barco Escape™ format for cinema.
But also when there is only a single projection screen in use in a nearly dark (for example cinema) environment, the contrast level of the projected image is affected by light returning via the walls, ceiling, floor and seats (e.g. with or without audience). In addition, national regulations may demand exit lights in the theatre to be visible all of the time. To achieve high dynamic range projection, these issues need to be tackled.
A projector screen with micro lenses or micro mirrors for the purpose of suppressing ambient light by means of a polarizer and quarter wave retarder, while focussing the light from a projector at a position where the light can escape from the structure and is optionally diffused is known (WO2016069631A1). A difficulty with this arrangement is that the position of the focus points depends upon the incident angle of the light coming from the projector, and therefore the screen has to be tailored for a certain projector configuration.
One dimensional structures could be used for the micro lenses or micro mirrors as these could simplify the production process. However one dimensional structures also have disadvantages. The acceptance of ambient light is only restricted in 1 dimension. Secondly the operation of the screen is symmetrical: all the light from the projector is scattered into the environment, all the light from the environment (that enters through the focus point) is returned towards the projector. With a 2 dimensional structure, if the projector is installed for the projected light beam not to be obstructed by the audience, the audience will not observe the ambient light entering through the focus point. With a one dimensional structure the light will return to the projector as a strip, and therefore a vertical lenticular structure will not work if the projector is behind the audience. As part of the light returning as a vertical strip will be picked up by the eyes of the audience, and a vertical band in the image will show degraded contrast performance. Only a horizontal lenticular structure could be considered in this case.
Methods to create micro-patterned anisotropic thin film layers such as retarders and polarizers have been developed based on photo-alignment technology. In a first step a Linear Photo Polymer (LPP) layer is applied on the substrate using a suitable coating technique after which the layer is dried at high temperature. A preferred alignment direction is induced in this layer by irradiation with linearly polarized light of an appropriate (UV or parts of the visible spectrum) wavelength. Liquid crystals in contact with a surface thus irradiated are oriented in accordance with this preferred direction. U.S. Pat. No. 4,974,941 describes such a photo-alignment method, that is reversible, by further irradiation of the layer with a second polarization direction, the alignment direction can be turned. U.S. Pat. No. 5,389,698 describes a photo-alignment layer that is non-rewritable.
When using the rewritable photo-alignment material and masking part of the surface during the second irradiation a patterned alignment can be achieved. Alternatively when a polarization pattern is applied to the illumination also with the non-rewritable photo-alignment layer a patterned alignment structure can be achieved.
Covering such a photo-alignment layer with a Liquid Crystal Polymer (LCP) layer, after which the layer is dried, annealed and UV-cross-linked enables the production of thin film anisotropic components such as retarders, as described in EP 0689084. The alignment and the layer thickness can be controlled to achieve the desired retardation. As such micro-structured retarders can be created. These processes have been successfully applied to create film patterned retarders for use in 3D TV sets. The process and the application for film patterned retarders are well described in the brochure for reactive mesogens from Merck.
The advantage of the photoalignment method is that it is a contact free method and multiple LPP/LCP layers can be applied on top of each other. Achromatic broadband retardance can be achieved by combining two planar LCP retarder layers. Further the angular dependency of the retarder can be reduced by using a bi-axial retarder (e.g. by stacking a C-axis retarder layer on top of a planar retarder layer).
By adding dichroic dyes to the LCP layer it is possible to create and pattern polarizer components. A process to create micro-polarizers has been described in U.S. Pat. No. 6,496,239.